species to their doom by mimicking both their sex pheromones, researchers
have discovered. The spider changes the blend through the night, to attract
species that are active at different times.

A bolas spider doesn't build a web. It slings a single line of silk across a
gap and waits on it, dangling another piece of silk with a sticky glob at the
end.

Next the spider releases chemicals that imitate the sex pheromones of female
moths. Male moths come in search of female company; the spider swings its silky
lasso and hauls in a meal.

Cunning, but limited. Different moth species produce very different pheromone
blends - not surprising, given the importance of mating with one's own species.
So spiders have to choose which species to attract.

And a combination of pheromones often turns a male moth off, even if his own
species is in the mix. So the spiders trade-off the effectiveness of their
mimicry of individual species against the number of species they can attract.

Let us prey

Different bolas spiders catch different combinations of moths. The American
bolas spider Mastophora hutchinsoni catches two unrelated moths - the
smoky tetanolita (Tetanolita mynesalis) and the bristly cutworm (Lacinipolia
renigera).

The two species fly at different times of the night. Cutworms are active -
and hunted - before about 22:30. Tetanolitas take wing from about 23:00 onwards.

Kenneth Haynes of the University of Kentucky and his colleagues shifted the
moths' body clocks so that spiders hunting one species met the other, and vice
versa.

The spider caught both throughout the night, showing that it produces a
mixture of the two species' pheromones, perhaps compromising its ability to
catch either one.

The spider also reduces the amount of cutworm pheromone as the night goes on,
the researchers found. Male tetanolitas don't like cutworm pheromone, Haynes'
team discovered. But male cutworms aren't discouraged by a whiff of tetanolita.

"To specialize on different moths at different times of the night is
amazing," says chemical ecologist Thomas Eisner of Cornell University, Ithaca,
New York. Hunting two species gives the spider some variety in its diet. But it
has still painted itself into an evolutionary corner, says Haynes.

"The evolutionary trajectory towards specialization seems a risky one," he
says. "In years when the prey is scarce, the spiders will have little food."
Bolas spiders are scarcer than web-spinners that catch a wider variety of prey,
Haynes says.

Moths' scaly wings allow them to slip out of spiders' webs. But
their pheromones are a weak point, says Eisner, as they are simple,
easy-to-manufacture chemicals. "Moths are liable to this kind of invasion of
privacy," he adds.

References:

Haynes, K. F.
et al. Aggressive chemical mimicry of moth pheromones by a
bolas spider: how does this specialist predator attract more than one species of
prey?
Chemoecology, 12, 99 - 105, (2002).